Portland cement: limestone is fired at 1450C, a process which frees CO2 from calcium carbonate to form calcium oxide, or quicklime. Gypsum is added, and depending on geography, a number of other additives such as fly ash, blast furnace slag, silica fume, various clinkers, sometimes metakaolin (to make it very white).
Strangely, cements are considered natural materials, I suppose because they are made of 'natural' mined minerals, such as limestone and bauxite. Now here is an interesting one: calcium sulfoaluminate low-energy cements require lower kiln temperatures, less limestone, thus less fuel consumption, less CO2 emissions, but 'significantly higher' SO2 emissions, which if I recall leads to acid rain.
Green cements using waste containing calcium, silica, alumni or iron, can replace clay, shale and limestone in the kiln, and other waste material can be used as fuel rather than coal or natural gas. It isn't clear if this produces cement that can be used for structural concrete.
Novacem, a research facility in the UK, has developed a magnesium silicate-based strong cement which absorbs CO2 as it hardens, making it carbon negative. Geologically speaking, limestone is very common throughout the world, supposedly so are magnesium silicates. Although one can develop a new carbon negative cement, getting it to replace existing, long-standing industrial processes is more difficult. Magnesium silicate is more commonly known as talc [Persian تالک] as in talcum powder: soft metamorphic rock, the main ingredient of soapstone.
But. but. and this is what I can't find, does the concrete made from all these different cements feel and look different from the energy consumptive Portland cement?